From Scientific American:
Black holes, as their name suggests, are dark. Perfectly dark. A black hole's gravity is so intense that beyond a certain boundary in its vicinity, known as the event horizon, nothing can escape. Not a rocket with its boosters on full blast nor a photon of light. Nothing. Despite the fact that astronomers cannot peer at what goes on inside the event horizon, a black hole's gravitational effects on its neighborhood allow for a number of indirect observations. Swirls of infalling gas heat up and give off radiation to illuminate a black hole's vicinity, and the orbits of stars around a black hole allow astronomers to estimate its mass. Now researchers have proposed a new optical technique to observe and study black holes by measuring the imprint they should leave on the light that passes near an event horizon.
A black hole's gravitational pull is so strong that it warps the spacetime around it. And if a black hole rotates, as would be the case for a hole that forms from the collapse of a spinning star, it drags spacetime along with it, a phenomenon known as frame dragging. (Less massive bodies also cause frame dragging on a smaller scale; NASA's Gravity Probe B launched in 2004 to measure the frame-dragging effects of Earth's rotation with sensitive gyroscopes.) According to a new analysis, the frame dragging of a black hole should put a detectable twist on nearby photons by imparting a trait known as orbital angular momentum. A light beam with orbital angular momentum looks a bit like a helix or coil when its component waves are mapped out. Whether any point along the beam is a wave peak, a trough or something in between depends on where that point lies with respect to the helix's central axis.